Treatment of molten light alloys



United States Patent 3,144,323 TREATMENT OF MOLTEN LIGHT ALLOYS AlbertR. Watson and Frederick H. Taylor, Birmingham,

England, assignors to Foseco International Limited, a

company of Great Britain No Drawing. Filed Apr. 27, 1960, Ser. No.24,907

Claims priority, application Great Britain May 1, 1959 3 Claims. (Cl.75- 67) This invention relates to the treatment of molten light alloys,by which term is to be understood magnesium, aluminium, alloys of thesetwo metals and alloys of either or both of these metals with othermetals in which alloys, magnesium or aluminuim or a mixture of these isthe predominant constituent.

It is well known that the foregoing light alloys, when melted, dissolvesubstantial quantities of hydrogen, originating for example from thedissociation of moisture from the atmosephere or introduced with thecommon additives to such metals. On solidifying the molten light alloyscontaining dissolved hydrogen, much of the hydrogen comes out ofsolution, giving rise to the presence of minute bubbles or porosity inthe solid metal.

It is common practice to attempt to remove dissolved hydrogen from lightalloys whereby an inert gas, for example, nitrogen or argon is bubbledthrough the molten metal before casting. A drying system for the inertgas is required and, even when the gases are thoroughly dried, lightalloys with very low hydrogen content are rarely obtained. Chlorine gasor chlorinated hydrocarbons which volatilise at the temperature of themolten metal have been used for the same purpose, but it has been foundthat large quantities of unpleasant fumes are produced during their use.

The present invention has for an object to provide a method of removingthe dissolved hydrogen from molten light alloys wherein the foregoingdifficulties are avoided or minimised. The operation is referred tohereinafter as de-gassing.

According to the present invention a method of degassing light alloyscomprises fiushing through the molten light alloys carbon monoxideand/or carbon dioxide. It has been discovered, rather surprisingly, thatdegassing with the oxides of carbon may be effected without giving riseto undesirable inclusions in the cast metal.

The oxides of carbon may be provided as such, e.g., supplied fromcylinders of the compressed gases or, in the case of carbon dioxide,supplied in the form of the solid (Dry Ice). These, however, are lesspreferred methods since the former requires the provision of cylindersand the latter involves a serious reduction in the temperature of themolten metal.

Preferably according to the invention the carbon monoxide or dioxide isgenerated in situ by adding to the molten metal a solid substance which,at the temperature of the molten light alloy, will decompose to yieldthe required gas, which then bubbles freely through the molten metal.

Thus for example, carbon dioxide may be generated in the molten lightalloy by adding thereto a carbonate which decomposes at the temperatureof the molten light alloy. Usually the alloy will be at a temperature inthe range 580 to 800 C. A suitable carbonate for this use is magnesiumcarbonate. If a carbonate is used which does not itself decompose toyield carbon dioxide at the temperature of the molten light alloy, itsdecomposition may be achieved by including with it a composition ofwhich the ingredients, at the said temperature, react exothermically(e.g., a mixture of aluminium and an oxidising agent therefor such asiron oxide), the temperature thus achieved by the exothermic reactionbeing suflicient to decompose the carbonate.

Alternatively a carbonate which does not itself decompose at thetemperature of the molten light alloy may be introduced in admixturewith a substance which will react with the carbonate, at the saidtemperature, to generate carbon dioxide. Suitable substances for thispurpose are alkali metal (e.g., sodium and potassium) silicofluoridesand borofiuorides.

Instead of a carbonate there may be used an oxalate, e.g., calcium orsodium oxalate. These oxalates decompose, at the temperature of themolten light alloy, to generate carbon monoxide and carbon dioxide.

In a further form of the invention the added substance is a mixture ofcyanamide and an oxidising agent (e.g., sodium nitrate). Such a mixture,at the temperature here under consideration, will generate carbondioxide with usually a proportion of carbon monoxide.

In a still further form of the invention the added substance is amixture of carbon or carbonaceous material (which is preferably finelydivided graphite but may be carbon black) and an oxidising agent, e.g.,a nitrate, peroxide, persulphate, chlorate or phosphate, preferably ofan alkali metal or less preferably of an alkaline earth metal. In thiscase it is convenient though not critical, to employ the carbon and theoxidising agent in substantially stoichiometric proportions.

Mixtures of any of these solid additives may be employed. To limit therate of the reaction, the additive compositions may contain a diluentwhich may be inert or may participate in the reaction to a limitedextent, e.g., sodium chloride, calcium fluoride, marble dolomite, zirconor grog (brick dust) but generally at least 5% of the additive willconsist of the carbon-generating substance or ingredients.

It is important that the moisture content of the additives be kept to avery low level. The moisture content of the additive is preferably notgreater than 1% of its weight and optionally does not exceed 0.25% ofits weight. It is accordingly often desirable to subject the additive todrying at a temperature below the temperature at which decompositionsets in, in order to reduce its moisture content to within the indicatedlimits. No difliculty appears to arise in the practice of the inventionfrom the dissociation of moisture in the ambient atmosphere. Especiallyin the case of carbon dioxide and mixtures rich therein, this may be duein part to a blanketing effect at the surface of the metal.

The compositions added to the molten light alloy according to theinvention may also include substances having other special effects onthe light alloy, e.g., grain refining agents or hardeners, e.g., saltsof titanium or boron or mixtures thereof may be so included.

The compositions serving as the additives according to the presentinvention may be introduced into the molten light alloy as powders orgranulated compositions, loose or in packets, e.g., paper packets, oraluminium containers, or may be preformed to tablet shape and added astablets.

Preferred compositions according to theinvention are those of which theactive ingredients are carbon, e.g., in the form of graphite, and anitrate, e.g., an alkali metal nitrate. It is found that the eflicacy ofthese compositions may be further improved by including in them a smallamount, e.g., up to 20 by weight of a fully chlorinated hydrocarbon,e.g., hexachlorethane, which appears to have a catalytic effect on theoxidation of the carbon and tends to reduce the formation of nitrogenoxide fumes.

It will be appreciated that the carbon oxide or mixture of carbon oxidesneed not necessarily be in a pure form. For example it may, whenproduced in situ, contain contaminants produced simultaneously, e.g.,oxides of nitrogen, or it may be diluted with inert gases such asnitrogen or argon in small proportions or in such proportions that theycontribute materially to the de-gassification.

The following are examples of compositions for use according to thisinvention:

Example I The following composition was made up from the granulated orpowdered ingredients as stated:

Parts by weight Sodium nitrate 36 Graphite 6 Hexachlorethane 3-5 Grog 30Sodium chloride 23-25 gms. per cc.

Example II The following composition was made up from the granulated orpowdered ingredients as stated:

Parts by weight Sodium nitrate 36 Graphite 6 Grog 30 Sodium chloride 28This composition behaved similarly to that of Example I but was a littleless vigorous in action.

We claim as our invention:

1. A method of treating molten light alloys consisting essentially of atleast one metal selected from the group consisting of aluminum andmagnesium to remove therefrom gases including hydrogen which methodcomprises generating within the body of molten metal a sufiicientquantity of at least one carbon oxide selected from the group consistingof carbon monoxide and carbon dioxide to remove said hydrogen from saidmolten alloy by adding to the molten alloy a mixture comprising carbonand an inorganic nitrate, said nitrate being present in at leastsubstantially stoichiometric proportion relative to the carbon.

2. A method according to claim 1 wherein the in0rganic nitrate is analkali metal nitrate.

3. A method according to claim 1 wherein the mixture includes an inertdiluent filler.

References Cited in the file of this patent UNITED STATES PATENTS1,318,074 Gowdy Oct. 7, 1919 2,160,812 Alden et al. June 6, 19392,281,216 Udy Apr. 28, 1942 2,362,507 Steinbeck et a1. Nov. 14, 19442,461,937 Strauss Feb. 15, 1949 2,583,533 Hiensch Jan. 29, 19522,654,670 Davis Oct. 6, 1953 2,952,534 Quinn Sept. 13, 1960 2,956,873Emmott Oct. 18, 1960 FOREIGN PATENTS 127,031 Great Britain May 29, 1919OTHER REFERENCES Carapella, Foundry Technology, Aluminum and Magnesium,August 1946 (pages 11 and 13).

Ser. No. 387,769, Lepp (A.P.C.), published May 4, 1943.

1. A METHOD OF TREATING MOLTEN LIGHT ALLOYS CONSISTING ESSENTIALLY OF ATLEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM ANDMAGNESIUM TO REMOVE THEREFROM GASES INCLUDING HYDROGEN WHICH METHODCOMPRISES GENERATING WITHIN THE BODY OF MOLTEN METAL A SUFFICIENTQUANTITY OF AT LEAST ONE CARBON OXIDE SELECTED FROM THE GROUP CONSISTINGOF CARBON MONOXIDE AND CARBON DIOXIDE TO REMOVE SAID HYDROGEN FROM SAIDMOLTEN ALLOY BY ADDING TO THE MOLTEN ALLOY A MIXTURE COMPRISING CARBONAND AN INORGANIC NITRATE, SAID NITRATE BEING PRESENT IN AT LEASTSUBSTANTIALLY STOICHIOMETRIC PROPORTION RELATIVE TO THE CARBON.